The present invention relates to the field of electrophotography and particularly to automatically controlling toner concentration in the two-component developer therefor. More specifically, the present invention relates to the xerographic process step of depositing toner on a latent electrostatic image carried by a moving photoconductor, to form a toned, reverse reading, visible image of an original document thereon, and to an improved means of controlling toner concentration, including means operable to add toner a maximum of once every M copies, and to inhibit the copying operation if the need to add toner is indicated N consecutive times, each time including no more than an M copy cycle.
In the xerographic process a toned visual image is transferred to a copy medium, for example to a sheet of copy paper at a transfer station. The toner is usually a pigmented thermoplastic resin. The individual toner particles are formulated such that they will soften under heat. When soft, they firmly stick to the surface of the copy paper. The amount of toner used in forming a copy is dependent upon the visual image content of the original document. Thus, maintaining an accurately controlled toner concentration requires a closed-loop control system which operates to sense actual toner concentration and to add toner when necessary.
Manual prior art systems for controlling toner concentration include a manually settable device for controlling the amount of toner metered to the developer material during each copy cycle. This method of control was based upon the operator's judgment. The operator observed the output copy quality and made a judgment as to whether more or less toner should be dispensed in order to improve the copy quality. Such systems necessarily depend upon the constant presence of the operator.
Various prior art automatic systems have been proposed for controlling toner concentration. These systems rely on measuring a physical characteristic of the developer material such as its electrical resistance, inductance, capacitance, or an optical characteristic.
The present invention provides an improved toner concentration control apparatus which, in effect, allows toner to be added as needed, and indicates a failure if the addition of toner is excessive.
In a specific embodiment of the present invention a first counter operates to integrate a signal indicative of a need to add toner. If, within a given time period, this counter reaches a high-count state, addition of toner is initiated, and the ability to subsequently sense a need to add toner is inhibited for a preset interval, such as a number of copy cycles. After this interval has expired, the ability to again add toner is enabled. A second counter is operable to count the number of uninterrupted, consecutive add-toner occurrences. If the second counter reaches a high-count state, the assumption is made that the copier is unable to properly tone the photoconductor's images, and further copying is inhibited.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawing.